Your current HDDs are 5400RPM, and your initial proposal would complement them with another 5400RPM, so the answer to your first question is yes.
Whilst Drobo works with HDDs of dissimilar rotation speeds, they also state that your Drobo would be best optimised with similar speed disks.
The answer to your next question is of course yes as well, however I would look for a 5400RPM models. But here’s the kick - if you only add 1 8TB, you will not get the full benefit of the disk capacity, as Drobo then reserves additional space for expansion. To show this graphically, go to:
-select your model and add in your current disks and then some hypothetical new ones.
To get the full benefit of any additional larger disk you need to add 2 of them - but please do it sequentially, allowing the Drobo to format and integrate the first one (to green) before adding the second. The Calculator will help you understand this better.
Good luck with your expansion, and please check your firmware is up to date beforehand.
Drobo actually do make some recommendations on types of HDD:
NOTE: read the fine print under each table, particularly those Drobo does not recommend.
As for mixing brands, most users do so without issue - I know that some users research their HDD brands for response, heat dissipation, noise etc. I haven’t yet mixed brands but will do so shortly, but I will still observe the same RPM across brands.